1. Field of the Present Invention
The present invention relates to a lithographic method and apparatus, and in particular to The method and apparatus for determining optical properties of a part of a lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
The lithographic apparatus comprises an optical system that includes an illumination system and a projection system. It is known that the projection system will have apodization properties. Apodization refers to a change in the intensity profile of an optical system. In some cases apodization may be introduced deliberately to achieve a desired outcome, in other cases it may be an inherent property of an optical system. In some designs of lithographic apparatus the apodization properties of the projection system are important and it is desirable to know what they are. Measuring or otherwise determining these properties is not straightforward and conventionally requires knowledge of the light distribution in the illumination field before the projection system. This can be difficult to determine and presents an obstacle to knowing the apodization characteristics of the projection system.